The beam divergence of an electromagnetic beam is an angular measure of the increase in beam diameter or radius with distance from the optical aperture or antenna aperture from which the electromagnetic beam emerges.

Typically, a piece of glass or plastic with curved faces that shapes light, but in flashlights the lens can also be the flat piece of glass or plastic at the front of the light that protects the light source.

A new standard for mounting auxiliary equipment such as telescopic sights, tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets to small arms such as rifles and pistols.

Also known as a MIL-STD-1913 rail, STANAG 2324 rail, or tactical rail, it is a bracket on some firearms that provides a mounting platform for accessories and attachments, similar to the Weaver rail mount.

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation, most popular being the visible & infrared which are ranges traditionally given as: ultraviolet light, 100-400 nm; visible light, 400-750 nm; and infrared, 750 nm-1 mm.

A TIR optic takes advantage of "total internal reflection" where light that strikes a surface at a shallow angle will bounce off the surface and continue through the material instead of scattering. The optic collimates the light and sends a concentrated beam of light out in the same direction, giving a tight hotspot with greater throw.